Method for focusing a double focusing mass spectrometer

ABSTRACT

A double focusing mass spectrometer utilizing an electric and a magnetic analyzing sector in tandem provided with a mechanically adjustable magnetic sector. Optimum focusing of the instrument is obtained by translational and rotational adjustments of the magnetic sector.

United States Patent Hull [54] METHOD FOR F OCUSING A DOUBLE FOCUSING MASS SPECTROMETER Charles W. Hull, Sierra Madre, Calif.

[73] Assignee: Bell & Howell Company, Chicago, Ill.

[22] Filed: May 7, 1969 [21] App1.No.: 822,610

[72] Inventor:

[52] US. Cl. ..250/4L9 ME Field ofSearch ..250/41.9 6,419

[56] References Cited UNITED STATES PATENTS R26,437 8/1968 Willdig ..250/4l.9

[ 51 June 20,1972

Marton ..250/4L9 X Robinson ..250/41.9

Primary Examiner-William F Lindquist Attorney-Christie, Parker & Hale [57] ABSTRACT A double focusing mass spectrometer utilizing an electric and a magnetic. analyzing sector in tandem provided with a mechanically adjustable magnetic sector. Optimum focusing of the instrument is obtained by translational and rotational adjustments of the magnetic sector.

5 Claims, 2 Drawing Figures Patented June 20, 1912 3,671,737

I N VEN TOR. C/Jdfl E5 W #044.

METHOD FOR FOCUSING A DOUBLE FOCUSING MASS SPECTROMETER BACKGROUND OF THE INVENTION The present invention relates to double focusing mass spectrometers and in particular to those of the tandem electric sector-magnetic sector type.

As with many kinds of precision instrumentation, it has been found that upon assembly, a double focusing mass spectrometer is subject to misalignments, manufacturing variations and the like causing the instrument to be slightly out of focus. There are two primary factors contributing to this result the first of which is that original design calculations utilized to determine the geometry and alignment of the instrument are normally not completely accurate and precise. The second factor is that in the course of manufacture and assembly of the instrument, the various components of the instrument are constructed with certain tolerances which result in a tolerance buildup which prevents the theoretical design geometry of the instrument from being completely met. Thus, in a newly assembled double focusing instrument, as well as other types of energy focusing instruments, the velocity and angular images of the resolved ion beam are displaced from the resolving slit and, in addition, are displaced relative to each other.

To eliminate this displacement of angular and velocity images from the resolving point, circuitry is incorporated in such instruments for providing compensating adjustments of the strength of the electric andmagnetic fields to which ions are to be subjected in passage through the analyzing regions. Auxiliary electric and magnetic fields can also be provided to complement or counteract the primary fields producing variations therein and consequent relocation of the position of the angular and velocity images. Such adjustments are not totally satisfactory in that they impose a requirement that substantial accessory equipment be built into the instrument, adding to its cost, making its assembly more critical and its operation more complex. In addition, magnetic and electric analyzing field adjustments typically involve interdependenteffects requiring a series of balancing adjustments of the various operating voltages of the instrument to achieve the desired focus.

SUMMARY OF THE INVENTION In contrast with the provision of additional electrical circuitry as outlined above to control and adjust the electric and magnetic fields within the mass spectrometer, the present invention provides a double focusing mass spectrometer in which mechanical adjustments accomplish the desired focusing while not being subject to the interdependent effects referred to above.

The invention provides a method of focusing a tandem electric sector-magnetic sector double focusing mass spectrometer for resolving a charged particle beam admitted into the spectrometer which comprises the following steps: A locus of 5 centers of rotation of the magnetic sector, generally in the plane of the ion beam, is determined, each of said centers having the characteristic that the location of the point of angular focus of the charged particle beam relative to a resolving point in the spectrometer is affected without affecting the location of the point of velocity focus of the charged particle beam relative to the resolving point when the magnetic sector is rotated thereabout. This locus of centers defines a substantially straight line. Once the location and direction of the straight line has been determined, the magnetic sector is then moved translationally in a direction parallel to said straight line, generally also in the plane of the ion beam, to locate the point of charged particle beam velocity focus at the resolving point of the spectrometer. Thereafter the magnetic sector is rotationally moved about any center of rotation lying on said straight line to simultaneously locate the point of charged particle beam angular focus at the resolving point of the spectrometer whereby the spectrometer is put into optimum focus.

The invention also provides a double focusing mass spectrometer comprising a source of ions, an object point for the ions, a resolving point for the ions and an ion collector located on the side of the resolving point opposite the ion source. An electric analyzing sector is located between the object point and the resolving point and a magnetic analyzing sector is tandemly located with respect to the electric sector between the electric sector and the resolving point. The object point, electric and magnetic sectors and resolving point define an ion beam path. Means for translationally moving the magnetic sector, preferably in the plane of the ion beam, are provided as are means for rotationally moving the magnetic sector preferably also in the plane of the ion beam whereby the ion beam velocity-and angular focus can be made to coincide and be located at the resolving point.

Electric sector-magnetic sector double focusing mass spectrometers are energy focusing instruments, i.e., they are capable of resolving a beam of ions which vary both in direction and velocity thereafter focusing ions of a predetermined mass (both angular and velocity images) at the resolving slit of the instrument. Thus, despite the fact that the energy of the ion beam may change, if the instrument is energy focusing, the focal point of the beam remains at the resolving slit. To determine whether the instrument is energy focused, the accelerating voltage is varied slightly causing a slight change in ion beam energy. If the ion beam still passes through the resolving slit, the instrument is in energy focus. To produce energy focus according to the present invention, the magnetic sector of the instrument is mechanically adjusted until such focus is obtained. In a somewhat analogous application, another type of mechanical adjustment of a magnet in a mass analyzer is shown in US. Pat. No. 2,794,126.

In practice, a double focusing instrument is set up by sweeping the ion beam across the resolving slit and utilizing a display device such as an oscilloscope to determine the exact operating conditions of the analyzer which produce a peak value on the display device for the particular mass under analysis. Once the peak has been located and centered on the resolving slit it will not move if the instrument is in optimum focus.

BRIEF DESCRIPTION OF THE DRAWING These and other features of the present invention will be better understood by reference to the following figures in which:

FIG. 1 is a layout of a tandem electric sector-magnetic sector double focusing mass spectrometer; and

FIG. 2 is an exploded view of the magnetic sector of FIG. 1 depicting other positions of the sector in phantom.

DESCRIPTION OF A SPECIFIC EMBODIMENT A typical double focusing mass spectrometer 10 as illustrated in FIG. 1 includes an object point 12, i.e., a point from which charged particles such as ions to be analyzed emanate, an electric analyzing sector (electrostatic lens) 14, a magnetic analyzing sector (magnetic lens) 16 and a resolving point, e. g., slit 18. The preceding elements define an ion beam path 20 interconnecting the object and the resolving points and passing between the electrodes 22 and 24 of the electrostatic lens and between the poles of a magnet, (the face of one of two such magnetic poles 26 is shown in FIG. 1), which comprise magnetic lens 16. Also shown in FIG. 1 is an arrow 28 which defines the locus of points or centers of rotation for magnetic sector 16, which points have the following particular characteristic. The magnetic sector can be rotated about such points such that when so rotated the point of ion beam velocity focus (velocity image, [3) does not move relative to the resolving slit while the point of ion beam angular focus (angular image, a) does. It has been further found that, to the first order, such points fall substantially on a straight line. The location and direction of the line can be determined empirically or by computations based on the geometry of the instrument, i.e., the entrance, deflection, and exit angles of the ion beam, the radii of curvature of the ion beam in the analyzing sectors and the spacing between the various slits and sectors of the instrument. For an instrument having the geometry shown in FIG. 1, line 28 defines the locus of such points of rotation and a particular point of rotation 30 is shown on this line.

The location and direction of the line of magnet translation is determined in the following manner. Twenty-one first order theory equations as disclosed in a l-Iintenberger and Koenig article titled Mass Spectrometers and Mass Spectrographs Corrected for Image Defects, page 16, Advance: in Mass Spectrometry, ed. J. D. Walden, Pergamon Press, 1958, are utilized. These equations relate the various double focusing analyzer parameters and can be solved to determine the ion beam width at any point in the system. According to the first order theory, focus is obtained when the first two coefficients of the several terms of the beam width equation are zero. These coefficients are functions of the position of the magnetic analyzing sector and change as the position of the sector is changed. The derivative of the first order coefficients is determined numerically thus determining the effect on the value of these coefficients for small changes in the position of the magnetic sector in the x and y directions as shown in FIG. 1 and for small amounts of rotation through an angle 0. An equation relating the incremental amounts of change (Ax, Ay and 6) in the magnetic sector position to the change in the two coefficients can then be derived. That portion of the equation relating to the second coefficient can be made to go to zero. Solving the resulting simplified equation, it can be shown that [2 i l l where AB is the change in the second coefficient of the beam width equation for given small changes Ax and Ay in the x and y direction, respectively, in sector position, and for a small angle 0 through which the sector is rotated; x and y are the coordinates of a line having the characteristic that the sector can be rotated about any point on this line without affecting the second coefficient of the beam width equation. This line also defines the direction of translation of the sector.

An exploded view of the magnetic sector 16 of the double focusing instrument of FIG. 1 is provided in FIG. 2. Instruments of the double focusing type are particularly useful in analyzing a heteroenergetic beam because such instruments resolve ions injected therein according to their mass despite the existence of angular and velocity variations in the ions of the beam. Thus a double focusing instrument provides both angular and velocity focus at the resolving slit for all ions admitted into the analyzing chamber.

An instrument having the elements shown in FIG. 1 is provided in addition with a source of ions, e.g., a spark source or surface ionization source located on the side of the object point opposite the electric sector and an ion collector located on the side of the resolving slit opposite the magnetic sector. Ions originating from the object point are resolved, i.e., sorted according to mass in the analyzing sectors and focused on the resolving slit. In the assembly of such instruments it has been found that when the various components are brought together within the instrument housing, it is normally not properly focused because of inaccuracies and errors in theoretical design calculations; and misalignments, displacements and angular variations due to manufacturing tolerance buildups. In addition, a double focusing instrument which has already been put into operation may become misaligned due to careless handling, etc. during use.

More specifically, improper instrument focus means that the ion beam angular focal point and the ion beam velocity focal point are displaced from the center of the resolving slit and from each other. Optimum focus is obtained when the ion beam angular and velocity image coincide with each other and are located at the center of the resolving slit and remain at that point when the energy of the ion beam is varied.

The foregoing is accomplished according to the present invention by providing a mechanically adjustable magnetic sector permitting predetermined modes of translational and rotational motion of the sector in a plane parallel to the plane of motion of the charged particle beam to be analyzed. Such mechanical adjustments are accomplished by providing a mounting for the magnetic sector which is arranged such that the sector can be moved back and forth in a predetermined translational direction in said plane. The magnetic sector mounting means are further arranged such that once the sector has been moved translationally and is fixed against further movement in that mode it can then be rotated in said plane about a center of rotation lying on the line defining the direction of translation.

For purposes of the present invention, points about which the magnetic sector of the instrument can be rotated are those which have the characteristic that when the sector is so rotated the location of the point of ion beam angular focus (angular image) is changed but the location of the point of ion beam velocity focus (velocity image) is not affected. For a given double focusing tandem electric sector-magnetic sector instrument having a given geometrical configuration, the locus of such points is a substantially straight line 28 to first order. This line defines the direction or mode of translational motion for the magnetic sector and phantom outline 34 depicts the location of sector 16 when displaced a distance A Q along line 28. The line of translational motion will, for various instrument geometries, be transverse to the direction of the charged particle upon entrance into the magnetic sector, parallel to said direction or perpendicular to said direction. Similarly, point 36 is a suitable point of rotation and when sector 16 is rotated thereabout by an amount 0, the sector is displaced to the position indicated by phantom outline 38.

Once line 28 has been located, the mounting means for the magnetic sector are arranged accordingly to permit translation of the sector along said line. FIG. 2 illustrates a possible set of circumstances to be encountered with a double focusing instrument. The point of angular focus is located at 40 and is displaced an amount A Ca from the resolving slit 18 in a direction toward magnetic sector 16. Similarly, the point of velocity focus 42 is displaced an amount A CB on the side of the resolving slit 18 in a direction away from the magnetic sector. By adjusting the magnetic sector an amount A Q and 6, the points 40 and 42 can be caused to coincide and be located at resolving point 44 located in the center of slit 18.

Once this is accomplished the instrument is in optimum focus and is ready for operation. The existence of optimum focus can be verified by adjusting the accelerating voltage to which the admitted ions are subjected. If optimum focus has been achieved, the beam remains focused at the center of slit 18. If the point of focus moves, focus has not been optimally set and additional adjustments according to the steps described above are required.

What is claimed is:

l. A method of focusing a tandem electric sector-magnetic sector double focusing mass spectrometer used for resolving a charged particle beam admitted into the spectrometer comprising the steps of:

1. determining a locus of centers of rotation of the magnetic sector, each of said centers having the characteristic that the location of the point of angular focus of the charged particle beam relative to the resolving point in the spectrometer is affected without affecting the location of the point of velocity focus of the charged particle beam rela tive to the resolving point when the magnetic sector is rotated thereabout, said locus of centers defining a substantially straight line substantially in the plane of the charged particle beam path;

2. moving the magnetic sector translationally in a direction parallel to said straight line and substantially in the plane of the charged particle beam path to locate the point of charged particle beam velocity focus at the resolving point of the spectrometer; and thereafter 3. moving the magnetic sector rotationally about a center of rotation lying on said straight line and substantially in the plane of the charged particle beam path to simultaneously locate the point of charged particle beam angular focus at lationally moving the magnetic sector produces motion of said sector in a direction parallel to the direction of the charged particle beam upon entrance into the magnetic sector.

5. A method according to claim 1 wherein the step of translationally moving the magnetic sector produces motion of said sector in a direction perpendicular to the direction of the charged particle beam upon entrance into the magnetic sector. 

1. A method of focusing a tandem electric sector-magnetic sector double focusing mass spectrometer used for resolving a charged particle beam admitted into the spectrometer comprising the steps of:
 1. determining a locus of centers of rotation of the magnetic sector, each of said centers having the characteristic that the location of the point of angular focus of the charged particle beam relative to the resolving point in the spectrometer is affected without affecting the location of the point of velocity focus of the charged particle beam relative to the resolving point when the magnetic sector is rotated thereabout, said locus of centers defining a substantially straight line substantially in the plane of the charged particle beam path;
 2. moving the magnetic sector translationally in a direction parallel to said straight line and substantially in the plane of the charged particle beam path to locate the point of charged particle beam velocity focus at the resolving point of the spectrometer; and thereafter
 3. moving the magnetic sector rotationally about a center of rotation lying on said straight line and substantially in the plane of the charged particle beam path to simultaneously locate the point of charged particle beam angular focus at the resolving point of the spectrometer whereby the spectrometer is put into optimum focus.
 2. moving the magnetic sector translationally in a direction parallel to said straight line and substantially in the plane of the charged particle beam path to locate the point of charged particle beam velocity focus at the resolving point of the spectrometer; and thereafter
 2. A method according to claim 1 wherein the rotational motion of the magnetic sector is confined to a plane parallel to the plane of the charged particle beam.
 3. A method according to claim 1 wherein the step of translationally moving the magnetic sector produces motion of said sector in a direction transverse to the direction of the charged particle beam upon entrance into the magnetic sector.
 3. moving the magnetic sector rotationally about a center of rotation lying on said straight line and substantially in the plane of the charged particle beam path to simultaneously locate the point of charged particle beam angular focus at the resolving point of the spectrometer whereby the spectrometer is put into optimum focus.
 4. A method according to claim 1 wherein the step of translationally moving the magnetic sector produces motion of said sector in a direction parallel to the direction of the charged particle beam upon entrance into the magnetic sector.
 5. A method according to claim 1 wherein the step of translationally moving the magnetic sector produces motion of said sector in a direction perpendicular to the direction of the charged particle beam upon entrance into the magnetic sector. 